The present invention relates to a communication system, a server apparatus, and a client apparatus for communication data with one another over a network, as well as to a reproducing apparatus and a reproducing method for reproducing content.
The progress of digital equipment in recent years has made it possible for diverse kinds of content such as music and images to be handled as digitized data (i.e., digital content). With the concurrent development of network technology, it has become commonplace to offer, distribute and sell such digital content over a network.
One characteristic of digital content is that in its digital form, the content is easy to copy without incurring deterioration in its quality. This requires making necessary arrangements for preventing illegal copy of the digitized data.
When digital content is to be delivered from a server to a client over the network, measures to prevent illegal copy are implemented illustratively as follows: the server attaches a license to the content of interest before sending it to the client. Upon receipt of the digital content, tamper-resistant modules of the client reproduce the content in keeping with constraints imposed on the content by the license. These measures are designed to prevent illegal reproduction of the content by a malicious user in a secure environment that provides license management over digital content (as discussed illustratively in Japanese Laid-open No. 2002-359616).
There can be a number of ways in which licenses impose restrictions on the digital content they are attached to. For example, a license may enable the corresponding digital content to be reproduced until a specific date and disable it from being reproduced thereafter. Another license may allow the digital content to be reproduced for 30 days following its download and prevent it from getting reproduced thereafter. A yet another license may inhibit the corresponding content from being reproduced until a particular date and enable it to be reproduced thereafter.
In each of these cases, the period of time in which the content of interest can be reproduced is limited. This requires the client to determine the validity of a given license in reference to a certain time base.
However, the time to be referenced is usually the time on the clock incorporated in the client itself. This is a potential security risk. The internal clock can be maliciously readjusted in order to reproduce content illegally beyond the reproduction period authorized by the license for the content.
The present invention has been made in view of the above circumstances and provides an apparatus and a method for preventing illegal reproduction of the content when its reproduction period is restricted by a license.